1. Field of the Invention
The present invention generally relates to flame retarded polyurethane foams, and particularly concerns flame retarded, low density polyurethane foams having improved color characteristics.
2. The Prior Art
When low density polyurethane foams are treated with flame retardants to reduce their flammability, certain physical properties of the foams are detrimentally affected. One of the frequent problems is scorching of the center of the foam buns which results in unacceptable color of the foam. This discoloration is reduced to an acceptable level pursuant to the present invention by incorporating phenothiazine and amine antioxidants into the foam.
It is known in the prior art to utilize phenothiazine to prevent propagation of free radical reactions that can cause scorch in polyurethane foams. Such reactions generally involve the ether moieties of the polyether employed in the foam formulation. Various compositions that are normally present in polyurethane foam formulations can catalyze these free radical reactions that cause the scorching. Among such compositions are tertiary amines which can be present in the formulation as catalysts for the reaction of isocyanate with water or with polyol, or as amine-started polyols or both, and various metallic compounds that can be present as impurities. These problems are described in U.S. Pat. No. 3,214,397.
The present invention is distinguished from the prior art in that the foams utilized according to the present invention do not become unacceptably scorched in the absence of the flame retardant. When the flame retardant is added to the foam formulation, however, unacceptable scorch results. It is a theory of the present invention that this scorching is the result of acidity produced by the flame retardant and by oxidation of the amines present in the foam formulation. This is because testing has shown that neither the phenothiazine nor the amine antioxidants alone give the excellent color results that their combination gives. it is a theory of the present invention that the phenothiazine acts as an acid acceptor and the amine antioxidants prevent oxidation of the amines, thereby preventing the unacceptable scorching of the foam by both scorching mechanisms.